C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Singleton class with static member variables          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message LinuxGuy Guest Posted: Wed Jun 15, 2005 10:40 pm    Post subject: Singleton class with static member variables Hi, I have come across singleton class with some member variables are declared as static with public scope. As singleton class always return only one instance. ie. single copy of object is maintained all the time. can someone tell me the reason behind declaring those variables as static one. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Timo Geusch Guest Posted: Thu Jun 16, 2005 8:23 am    Post subject: Re: Singleton class with static member variables LinuxGuy wrote: Quote: I have come across singleton class with some member variables are declared as static with public scope. That doesn't sound like a very good idea to me, but it's hard to determine this without looking at the actual source code. What were those variables and how did they relate to the Singleton itself? Quote: As singleton class always return only one instance. ie. single copy of object is maintained all the time. can someone tell me the reason behind declaring those variables as static one. Gissa some source code and we may be able to figure it out... [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Greg Guest Posted: Fri Jun 17, 2005 1:34 pm    Post subject: Re: Singleton class with static member variables LinuxGuy wrote: Quote: Hi, I have come across singleton class with some member variables are declared as static with public scope. As singleton class always return only one instance. ie. single copy of object is maintained all the time. can someone tell me the reason behind declaring those variables as static one. One likely reason for declaring data members of a singleton static is to avoid tying the lifetime of those variables to the lifetime of the singleton object itself. In fact if the singleton has no special initialization nor termination requirements, it's often easiest to dispense with the singleton object completely and declare all of its data members and methods as static. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Antonio Maiorano Guest Posted: Mon Jun 20, 2005 9:06 am    Post subject: Re: Singleton class with static member variables Quote: As singleton class always return only one instance. ie. single copy of object is maintained all the time. can someone tell me the reason behind declaring those variables as static one. One reason this is done is to avoid having those variables created on the heap. This can save on the expensive creation time of the singleton, and can also improve cache coherency and alignment by having the compiler position the memory blocks in static memory. Antonio Maiorano Software Engineer Electronic Arts - Montreal [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Mon Jun 20, 2005 9:17 am    Post subject: Re: Singleton class with static member variables Greg wrote: Quote: LinuxGuy wrote: I have come across singleton class with some member variables are declared as static with public scope. As singleton class always return only one instance. ie. single copy of object is maintained all the time. can someone tell me the reason behind declaring those variables as static one. One likely reason for declaring data members of a singleton static is to avoid tying the lifetime of those variables to the lifetime of the singleton object itself. In fact if the singleton has no special initialization nor termination requirements, it's often easiest to dispense with the singleton object completely and declare all of its data members and methods as static. I'd say that typically, the arguments concerning object lifetime work in the other direction. If someone calls Singleton::instance(), or anything that accesses one of the static variables, from the constructor of a static object in another translation unit, he has no guarantee that the static members have been constructed. Of course, if one can be sure that the class will not be used in constructors of static objects, it may be useful that the static members will be constructed before the first call to Singleton::instance(). I've also used static members at times to ensure that the singleton object itself was constructed before entering main (and thus, before the initialization of a multithreaded context). -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze@gabi-soft.fr Guest Posted: Mon Jun 20, 2005 2:07 pm    Post subject: Re: Singleton class with static member variables Antonio Maiorano wrote: Quote: As singleton class always return only one instance. ie. single copy of object is maintained all the time. can someone tell me the reason behind declaring those variables as static one. Quote: One reason this is done is to avoid having those variables created on the heap. This can save on the expensive creation time of the singleton, Creation time of a singleton is, almost by definition, irrelevant, since it only occurs once. Quote: and can also improve cache coherency and alignment by having the compiler position the memory blocks in static memory. That depends a lot on the implementation. The linkers I know don't do anything to ensure cache alignment. Nor does operator new, but there are generally options with malloc to do so. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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